The flight control surfaces and engine thrust of an aircraft are trimmed to maintain a desired flight condition described by altitude, speed, attitude and heading of the aircraft, as well as to effect some desired changes in altitude, speed, attitude and heading. Typically, the flight control surfaces are trimmed to provide straight and level flight.
Trim may be used to compensate for such factors as imbalances or shifts in load (people, cargo, and fuel) and for winds that would otherwise undesirably alter the altitude, speed, attitude or heading of the aircraft and for thrust asymmetry due to engine failure. For example, trim may be used to compensate for an imbalanced in the weight distribution of cargo that would otherwise cause the aircraft to roll undesirably to one side. Similarly, trim may be used to compensate for a cross wind that would otherwise cause the aircraft to yaw undesirably away from its desired heading.
More particularly, when the aircraft is tending to roll to one side, ailerons and spoilers may be moved to a position that returns the aircraft to level flight. Similarly, if the aircraft is being blown off course, a rudder may be moved so as to maintain the desired heading.
The control surfaces on smaller aircraft may be positioned manually, such as by using pilot operated mechanical systems, e.g., cables and pulleys. However, larger aircraft generally require electric motor or hydraulically operated control surfaces. On some aircraft, such as those with augmented (fly-by-wire) flight control systems, an automated trim system attempts to maintain desired aircraft altitude, speed, attitude and heading with minimal pilot intervention.
Flight simulators are important tools that are extensively used by the airline industry and the military for pilot training and disaster simulation and by aircraft manufacturers for aircraft development. One of the most important aspects of flight simulators is the trim system and method.
Although contemporary trim systems have proven generally useful for their intended purpose, such contemporary systems do suffer from inherent deficiencies that detract substantially from their utility and desirability. This is particularly true for modern aircraft that have highly nonlinear and highly coupled dynamics. It is also particularly true for flight control systems that are augmented.
This is also particularly true when such system follows the p-beta control law, which introduces highly augmented systems and significant system couplings between roll/yaw control and roll/yaw motion. As a result certain trim variables have switched their influences on aircraft motion from one axis to another axis, which has makes it impossible for contemporary trim systems to adjust trim variables.
Trim systems determine trim variables such as angles of attack and sideslip, flight path angle, column input, pedal input, wheel input, and throttle position to drive both angular accelerations (pitch, roll and yaw) and point mass linear accelerations (tangential acceleration and centripetal acceleration) to target values (which are typically zero).
However, the contemporary trim process is a single-axis process. That is, the trim variables are determined one at a time to drive a single acceleration corresponding to a single trim variable. For example, sideslip angle may be used to drive yaw acceleration to zero (thereby maintaining a desired heading). After yaw acceleration has been driven to zero, then a different trim variable may be used to drive its corresponding acceleration to zero, and so on.
In this manner, contemporary trim systems solve for one trim variable by driving one acceleration to its target value at a time. After the trim system determines a trim solution for a first axis or acceleration, then the trim system moves on to a second axis and thus trims the second axis using a different trim variable. However, once trim is achieved along the second axis, the previously trimmed acceleration along the first axis typically becomes untrimmed. Thus, the first axis must be re-trimmed.
The previously trimmed acceleration along the first axis typically becomes untrimmed after trim is achieved along the second axis because of strong couplings of the dynamic systems involved. This undesirable effect necessitates iterative trimming, which is inherently inefficient. That is, each degree of freedom must be repeatedly re-trimmed until all degrees of freedom are within desired tolerances. Such iterative trimming is not desirably robust and often fails to produce a trim solution when the aircraft is near the boundaries of the flight envelope and flight control limits.
One of the flight boundaries is the lowest speed at which an aircraft can remain trimmed with both pedal and wheel input at their mechanical limits. When aircraft is nearing this speed limit at maximum pedal input, the wheel becomes ineffective in controlling rolling moment. Sideslip angle can still effectively control the accelerations in both roll and yaw axes. The contemporary trim systems often fail to find the trim solutions at this boundary.
Further, although the p-beta control laws of some modern aircraft eliminate the lateral acceleration dependency on bank angle, they add a yaw acceleration dependency on the bank angle. This new acceleration dependency shift makes contemporary single axis trim process ineffective.
As a result, there is a need for a trim system that generally simultaneously determines values for trim variables in a plurality of different degrees of freedom so as to substantially mitigate the iterative aspects of contemporary trim systems and thereby enhance efficiency and provide a more robust trim system.